Popular Science Monthly/Volume 81/August 1912/Cold Storage Problems
|COLD STORAGE PROBLEMS|
By Dr. P. G. HEINEMANN
UNIVERSITY OF CHICAGO
COLD storage is but one of many methods of conserving foods. There is a period of plenty for every food substance, during which time much would go to waste if there were no provision for conservation, and when prices are low to the consumer and profits large to the producer. During the balance of the year food is scarce and must of necessity bring exorbitant prices, if within reach at all. Usually the period of plenty covers but a fraction of the year and bears to some extent a definite relation to climatic and soil conditions. The gap between the periods of plenty and scarcity is felt least in the centers of population, where transportation facilities make many foods available throughout the year. There, for instance, berries are obtainable before and after the natural period of supply has expired, the provisions being carried in refrigerator cars from southern and northern climates, where conditions favor their production. Also many fruits, such as oranges, grapefruit and pineapple, are carried successfully from tropic and subtropic countries to parts of the world where they can not be grown. These conditions are made possible by efficient transportation. Undoubtedly within the near future transportation facilities will be improved so as to encourage the raising of crops in tropical climates to such an extent that fruits and vegetables will be available in northern countries throughout the year. Many luxuries of the table could not now be obtained, were it not for the systematic conservation of food articles.
There are more than 3,000 million dollars worth of foods placed in cold storage annually, of which about one half is meat. Capitalists are expending money and scientists are giving their time to the exploitation of this promising field. Although the problem is still in its infancy, ways and means of furthering cold storage are being investigated. The chief problems to be investigated are the handling of foods previous to placing them in cold storage; the chemical changes taking place during storage; the study of the microorganisms that produce these changes; the sanitary conditions of cold storage warehouses, and the care of foods after leaving cold storage.
Many foods that constitute a regular and almost necessary part of the diet of civilized man are really nothing but the results of preserving perishable products. As milk, for instance, perishes within a short time, the nutritive constituents have been transformed for ages into cheese, butter, fermented beverages, etc. During recent years milk and eggs have been evaporated or desiccated. Their keeping qualities are thus enhanced and by adding the requisite amount of water they-can be restored to nearly their original condition. Frozen milk, which, when thawed, represents milk almost equal to the fresh product, is being exported from Denmark in large amounts.
In a crude and empirical fashion many substances have been subjected to preserving agencies for ages. Savages have desiccated meat, or preserved it by immersion in sour milk. The farmer slaughters in the fall and keeps the meat in his cellar, where low temperature and the drying out of the outer layer protect the meat from serious decomposition. Fresh fodder for cattle is fermented into silage and grapes into wine. Acids for pickling vegetables, fruit and meat; sugar in concentrated solution for preserving berries and fruit; these means have long been utilized. High and low temperatures also have been used extensively for preservation with success. These methods have been reduced by science to certain fundamental principles, which may be considered conveniently under five heads: (1) Harmless preservatives. (2) Chemical preservatives. (3) Heat. (4) Desiccation. (5) Low temperatures.
These methods to prevent "spoiling" of foods were practised to some extent before the causes of food decomposition were understood. With the dawn of the science of bacteriology it was learned that decomposition of organic matter was due almost exclusively to the vital activities of microorganisms. It became clear that the sole end was the destruction or restraint of development of these minute organisms. Microorganisms require food, but this food must be diluted with water, consequently the abstraction of water by desiccation or freezing, or by boiling in concentrated sugar solutions will restrain their multiplication. Some chemicals are poisonous to bacteria, but are used only to a limited extent on account of possible injury to the consumer. High temperature kills microorganisms rapidly, low temperature slowly, and below the freezing point there is little, if any, multiplication. It must be distinctly understood that in the absence of microorganisms many years must elapse before organic matter can show any signs of decomposition. If this fact is borne in mind it will readily be understood that, if microorganisms are restrained from activity or destroyed, no matter by what means, there can be but little change in food substances. Antidiluvian animals have been disinterred from a bed of ice, where they were buried for ages, and the flesh has been found to be still in good condition.
The chief object of this article is the consideration of effects of low temperatures; the other methods will therefore he mentioned but briefly.
The Use of Harmless Preservatives
Sugar and acids are perhaps the most important of these. By adding large amounts of sugar to perishable foods, such as berries or their expressed juice, and dissolving the sugar by heat, the berries may be kept in good condition for unlimited periods. The same result is obtained by the use of acids in the preparation of pickled vegetables, pickled meat, the curing of hams, etc. Spices also act as preservatives and many delicacies are preserved in oil.
The Use of Chemical Preservatives
Of these benzoate of soda, formalin, boric acid and salicylic acid are used to some extent. Whether these are unwholesome in the small quantities used is still disputed. The relative amounts are certainly very small and harmful results can only be incurred by continued use. Alcohol is also a powerful preservative and is produced by allowing fruits, etc., to ferment. The perishable grape is thus transformed into stable wine.
The Application of Heat
The enormous canning industry uses heat as a preservative, followed by exclusion of air. First the microorganisms are destroyed by heat and then renewed invasion is prevented by sealing the containers hermetically. Temperatures, lower than those used for canning, are applied in pasteurizing milk and beer. Pasteurization is carried out by heating milk or beer from 140 to 145° F. for 20 to 30 minutes. This process does not kill all microorganisms, but the great majority, including disease germs. If pasteurized milk is cooled rapidly and kept cold it will remain sweet for days. Pasteurized beer will keep much longer, since the amount of alcohol contained in it aids in protecting against decomposition.
Conservation by Desiccation
Desiccation—the abstraction of water—has been applied to the preservation of fruits, meat, eggs and milk. Fruits are dried in the open air exposed to the sun or by artificial heat. Desiccated products, of course, can be of no better quality than the original substance. Poor fruit, dirty milk, decomposed meat and aged eggs can not be expected to improve by desiccation. Milk should be clean before desiccation, but unfortunately this is not often the case, and the production of eggs is not as sanitary and well regulated as might be desired. Eggs are rarely, if ever, sterile when freshly laid, the term sterile applying to the total absence of microorganisms. It is certain that, after laying, bacteria can penetrate the shell, especially if the shell is moist. These bacteria find excellent food in eggs and will multiply at an enormous rate unless the eggs are kept cold. The producer usually brings the eggs once a week to the country store, where they remain for another week or two before being delivered to a factory for desiccation. When we consider that one bacterium may have a progeny of 17 millions in 24 hours under favorable food and temperature conditions, it is not surprising to find that eggs used for desiccation contain millions of microorganisms. Bad eggs are usually recognized by holding them up to a bright light in a dark room, a process known as "candling." This will reveal dark spots resulting from bacterial multiplication, or colonies of molds, or embryonic development of the chick. But we must remember that millions of bacteria take up a smaller space than the head of a pin, so that the process of candling will show their presence only after enormous multiplication has taken place. On the other hand, if fresh eggs are desiccated within a few hours or a day after laying and have been kept cold during this interval, the final product will contain about the same number of bacteria as the original eggs. It must not be assumed, however, that the presence of many millions of bacteria is necessarily injurious to health. Of all species of bacteria known to science an exceedingly small number is injurious, and these are rarely found in eggs.
Conservation by Cold Storage
The present methods of cold storage are the natural result of evolution from the practise of using the cellars of farmers and produce dealers. Surplus food material is stored in warehouses, where it will keep in good condition for various periods of time, these periods depending largely upon the degree of temperature maintained. Such storage makes possible an artificial season, which may be long enough to bridge the gap between one producing season and the next. The importance of this can not be overestimated and has found expression in the fact that the Canadian government is subsidizing cold-storage plants.
The degree of temperature to which foods are exposed in cold storage varies according to the nature of the food. Fruits, vegetables and and shell eggs are permanently injured by freezing and are therefore kept just above the freezing point. If eggs are broken, the yolks and whites mixed and then frozen, they can be preserved for a long time. What has been said about desiccated eggs applies with equal force to frozen and cold storage eggs. If they are in good condition when placed in cold storage or when broken for freezing they will keep for a long time. Usually considerable deterioration takes place before they reach the packer. Especially is this true of summer eggs. April and May eggs, if placed in cold storage in good condition, are of better quality than so-called fresh eggs during the summer months. The popular prejudice against cold storage eggs is not entirely unjustified, since eggs are sometimes stored after having become tainted. As a rule, they are well taken care of after they reach the packer, while the producer uses no precautions to preserve their original condition. It is probable, that there are more tainted eggs sold, that have never been in cold storage, than tainted cold storage eggs. Microorganisms multiply slowly, if at all, at the freezing point. Scientific investigations have not settled this point satisfactorily. Some bacteria may survive at very low temperatures, but freezing gradually kills most of them, as has been demonstrated by bacteriological studies of ice. It has been shown also that the number of bacteria in eggs in cold storage decreases constantly to a point where but few are left. Some observations are on record which seem to show that bacteria may multiply slowly near or below zero. This subject needs more thorough scientific investigation. However, it is safe to say that eggs placed in cold storage in good condition will remain practically unchanged for many months.
Meat, poultry and fish are stored in large quantities in the frozen condition. It is perhaps not generally known that cattle are most plentiful during the three months of late summer and early fall, although there is a limited supply during the remainder of the year. The packers buy all the cattle obtainable, slaughter it, supply the immediate demand and place the surplus in cold storage. When the season of plenty has expired they can draw on cold storage stock and keep the market supplied. Thus prices are kept more or less balanced during the entire year. This is of inestimable value. The prices remain within the limits of every one and a plentiful supply can be carried over from one season to the next without allowing any to go to waste. The meat inspection service carried on by the government is well organized. Every piece of meat handled by the packer has a definite record and consequently only fresh meat is placed in cold storage. It is authoritatively stated that federal or state inspection of eggs and fruits destined for cold storage would be welcomed by all reputable packing houses.
Packers usually have three warehouses, a different temperature prevailing in each one. The fresh meat is hung in the "chill room" at a temperature of 32° F. The temperature gradually rises to about 40° F. while the room is being filled with fresh warm meat. The next morning the temperature is down to 32° F. again. The meat is then removed to the "freezer." Here a temperature of—9 to—12° F. is maintained and the meat left for two to four days. It is now frozen throughout and is transferred to the cold storage room and kept here at 12 to 15° F. This temperature is low enough to keep meat in a frozen condition. Poultry and fish are treated in a similar fashion.
The handling of poultry for cold storage is not as well regulated as that of meat. A large amount of poultry is killed on the farm, instead of at the packing house. From the farm it may be shipped by two methods, the "wet" and the "dry." If shipped "wet," the poultry is packed in ice and the skin becomes wet and softened, so that bacteria can penetrate readily. If shipped in modern refrigerator cars, which are recharged with ice every day, the poultry remains "dry" and arrives at the cold storage warehouse in good condition. Poultry is frozen either with the entrails (undrawn poultry) or after the entrails have been removed (drawn poultry). Formerly it was thought that the presence of the entrails had an injurious effect upon the keeping qualities and in some places ordinances have been framed prohibiting the shipment of undrawn poultry. Recent investigations have shown conclusively that undrawn poultry keeps in better shape than poultry drawn in the ordinary fashion. The fundamental cause for the difference in keeping qualities of drawn and undrawn poultry is the presence of bacteria in large numbers in the entrails. In the ordinary process of drawing, of which there are different methods, the membranes of the body cavity can not escape injury and bacteria invade the muscles. The membrane remains intact if poultry is not drawn, or drawn carefully. For practical purposes it is no doubt preferable to allow the entrails to remain until the poultry is prepared for consumption. Recently poultry has been frozen in paraffined pasteboard boxes, which are delivered to the consumer in a sealed condition and with proper directions for thawing. Frozen poultry should be thawed slowly in a house refrigerator, which process requires about two days.
The handling of fish for cold storage is surrounded with difficulties. Fish is not part of our daily diet as meat is, but is consumed largely on one day of the week. The fisherman may have a large haul or a small haul. If large, there will be surplus to be disposed of. A number of fish are frozen together, forming cakes of about 20 pounds each. The cakes are piled up on top of each other in a cold storage warehouse. The ice hermetically seals the fish, but every well regulated cold storage warehouse has some provision for ventilation and the moving air takes up the evaporating ice from the free parts of the cakes containing the frozen fish. The heads become exposed and are recovered with ice by periodical sprinkling and dipping. This requires considerable attention on the part of the warehouse man, but if properly carried out will preserve fish for long periods. Thus waste is prevented and the market kept supplied.
Changes taking Place in Meat, Fish and Poultry in Cold Storage
The changes taking place in meat, poultry and fish during cold storage are both of physical and chemical nature. The physical changes are chiefly those which are due to freezing and thawing. More than one half of the muscle substance is water. When the temperature of water is lowered the volume is reduced, but below 39° P. increases again. Therefore, when the meat is chilled below 39° F. the water contained in the muscle cells filters through the cell membrane and fills the interstices between the layers of cells. Here it freezes when the meat is placed in freezer storage. Gradually the interstices are filled with crystals of ice and the cells are squeezed out of shape. If the meat is thawed the result will be different according to whether the thawing process is carried on slowly or rapidly. If slowly, the water will filter back into the cells and they will assume, nearly at least, their normal shape, and the meat will, to all outward appearance, look like fresh meat. If the meat is thawed rapidly, considerable juice will ooze out and the cells will never resume their normal condition. It is better, therefore, to thaw meat slowly.
Some of the water contained in the skin naturally evaporates during cold storage, so that the skin shrivels. To restore the natural appearance and replace the lost weight, frozen poultry is frequently soaked in hot water. This softens the skin and favors the invasion of bacteria. Poultry treated in this fashion will deteriorate rapidly and the practise should therefore be condemned. The physical changes described do not injure the general appearance, flavor and digestibility of the meat, some investigators claiming that both flavor and digestibility are favored.
Whatever chemical changes take place in meat, poultry and fish in cold storage are chiefly the result of the activity of microorganisms (bacteria and molds). Since, however, bacteria multiply slowly at the freezing point, and probably not at all in frozen meat, the changes are slight. At 32° to 40° some changes are noticeable after 15 days' storage. These changes are also slight and are restricted not only by the temperature, but also by the layer of dry meat forming on the outside, which makes it difficult for bacteria to penetrate the interior.
The fat is probably not attacked by bacteria, but undergoes a slow oxidation under the influence of light and the oxygen of the air. By this process the acidity of the fat increases, but the change is very slow, if the meat is kept frozen, and is not noticeable for months. It has also been claimed that there is some change in meat due to the action of ferments, which may be contained in the muscle cells. This process is called "autodigestion." There is, however, no reliable evidence of this; in fact, new investigations seem to show that bacteria are the only agencies worthy of consideration in the decomposition of meat. Ferments, moreover, do not act at very low temperatures, although they are not destroyed and resume activity when meat is thawed.
Whether bacteria multiply at freezer temperature in liquid foods is problematical. It is well established that bacteria die gradually in ice so that after several months ice is practically sterile. Ice, however, offers little nourishment for bacteria, while meat contains plenty of food, which is available if water is present. Bacteria can not multiply in solid substances and therefore multiplication in frozen meat is impossible. It is reasonable to assume that bacteria will die in frozen meat as they do in ice. Conditions of frozen fish are quite similar to those of meat and poultry. A few investigations of this problem are available and, in substance, agree with the findings in frozen meat and poultry. The bacteriological aspect of cold storage meats is largely speculative and scientific investigations are much needed to throw light on the problems of cold storage.
In a general way the conclusions reached by those investigators who have worked on cold storage problems are that there is no appreciable difference in chemical composition between fresh meat and meat kept frozen for periods longer than two years. No progressive changes could be determined with precision. In regard to frozen poultry it has been stated that the changes in chickens in 24 hours at 65 to 75° F. are greater than in cold storage at 10° F. for 12 months. The changes taking place in cold storage for 12 months are comparable to storage in a house refrigerator for 5 days, or in the packer's chillroom at 32° F. for 15 days, this not being sufficient time to bring about the tenderness and flavor of so-called "ripened" meat. Most chickens bought in the market as fresh are of poorer quality than chickens frozen for 12 months, and are poorer than many kept in cold storage for 16 months.
The conclusion is inevitable that cold storage at 10° to 15° F. has no deteriorating effect on the condition of meats, poultry and fish for a period long enough to bridge over the time from one flush season to the next, that is to say, for about 12 months. The term "deterioration" is a vague one and is interpreted largely on the basis of individual opinion and taste. Some foods are not considered fit to eat unless they have "ripened," but the ripening process is nothing but a decomposition brought about by bacteria and molds. Cheeses of all kinds are permitted to ripen for various periods of time and the changes taking place during the ripening process are comparable from a chemical point of view to the so-called deterioration of meat. The most delicate and savory steaks are subjected to a ripening process, which consists in storing them for 30 to 60 days at a temperature of 32°-40° F. During this time the muscle fibers undergo changes, which render them more soluble and consequently more tender, palatable and digestible. This is practically the same process as the ripening of cheese, with this distinction, that decomposition of cheese is allowed to go farther than decomposition of meat. It is also true, that the tastes of individuals differ largely. Game is usually considered best when decomposition has gone far beyond that of ripened meat, and when it has developed the so-called "gamey" flavor, which is the result of progressive decomposition. Putrid eggs are preferred to fresh eggs by the Chinese, and rancid butter to fresh butter in some tropic countries. Some cheeses are most palatable to many people when decomposition has gone so far as to produce decay. The cheese then contains considerable amounts of ammonia, a sure sign of advanced putrefaction. Virginia hams after two years' storage are considered at their best by connoisseurs.
It can readily be seen that it is difficult to establish a limit in a process and call it ripening, while the same process continued for some time would be called deterioration. All standards to-day are arbitrary in the light of our knowledge and the judgment of health officers is seriously taxed. It is obvious that it is largely a matter of opinion, and it is certain that much investigation, chiefly from a bacteriological point of view, is necessary before intelligent legislation can properly take care of cold storage. Recently a bill was introduced into the federal senate, which proposes to limit the time of cold storage of beef to 7 months; of pork and mutton to 4 months; of poultry, game, fish, eggs and butter to 3 months. By such limitation the very purpose of cold storage is defeated and, in the light of our knowledge of cold storage problems, entirely unjustified. Regulation of commercial cold storage should be attacked from two standpoints: (1) Cold storage warehouses should be subject to government inspection as to construction, ventilation, temperature and sanitary conditions. This inspection should also be extended to refrigerator cars. (2) All foods destined for cold storage should be inspected as meat is inspected at the present time.
Poultry should be placed in cold storage only when fresh and the killing should be done by the packer. It should be stored undrawn. Under a system of inspection there is no reasonable doubt but that food coming out of cold storage would be in as good condition as when going in. Legislation limiting cold storage is entirely out of place. It is true, that foods coming out of cold storage decompose more rapidly than fresh foods, not because—as has been suggested—fresh food is still "alive," but because the physical structure has been changed to some extent by freezing, rendering the food more accessible to bacterial invasion.
The cold storage warehouse is an all-important asset to modern economy, but the facilities are limited at present. It has been stated that in Greater New York about two million pounds of meat are kept in cold storage, while the weekly consumption of meat is 80 million pounds. If a storm or some other calamity should prevent the shipment of fresh meat the cold storage supply would last but a short time and a meat famine would follow.
It seems clear that cold storage is a great boon to man at present and will be of greater value in the future. Not many years ago, before cold storage facilities were generally available, it was not an uncommon occurrence to buy tainted meat from recently slaughtered animals. The animals had to be killed in places where flies were common and the meat offered favorable breeding places for the pests. Modern sanitary slaughtering houses with the addition of cold storage facilities have conquered many evils of similar nature. The cold storage industry should receive the hearty support of the public and it should be the aim of legislators to protect legitimate cold storage by appropriate regulations. This can not be brought about until the great problems involved have been thoroughly investigated both from a scientific and commercial point of view.